The present invention relates to a process for the preparation of methylglyoxal dimethyl acetal (1,1-dimethoxy-2-propanone) by reacting methylglyoxal (2-oxopropanal) with methanol in the presence of an acidic catalyst, in particular of an acidic ion exchanger.
In the previously described preparations of methylglyoxal dimethyl acetal (MGDA) from methylglyoxal (MG) and methanol, it was frequently attempted to remove the water, which can originate both from the starting methylglyoxal solution and from the reaction itself, from the mixture by means of entraining agents during the reaction (eg. Braude et al., J. Chem. Soc. 3328 (1955); Hoechst AG, DE 29 47 383). This requires the use of readily volatile, generally toxic assistants, for example benzene, which may contaminate the product. Moreover, the yields described are only 67% at the most. 1,2,2,2-Tetramethoxypropane is described as a waste product.
Furthermore, a Japanese Patent Application (JP 59199651A) describes the acetalation of methylglyoxal with methanol over ion exchangers in suspensions, working up being effected by means of extraction with methylene chloride as the solvent. Since methylglyoxal dimethyl acetal is largely used for the preparation of pharmaceutical products, the use of this toxic assistant appears unacceptable. No information is given with regard to the whereabouts of the ion exchanger and the distillation residues. In addition, our own experiments (cf. Comparative Example) show that the yields stated in JP 59199651A cannot be repeated with the methylglyoxal available in industry (about 40% strength aqueous solution). The methanol excess of 1:23 used in the Examples in this publication and the long reaction times of 16 hours are moreover disadvantageous for an industrial methylglyoxal dimethyl acetal synthesis.
It is an object of the present invention to remedy the abovementioned disadvantages in the preparation of methylglyoxal dimethyl acetal from methylglyoxal and methanol.